Temperature controlled short duration ablation

ABSTRACT

A method, including selecting a first maximum radiofrequency (RF) power to be delivered by an electrode within a range of 70 W-100 W, and selecting a second maximum RF power to be delivered by the electrode within a range of 20 W-60 W. The method also includes selecting an allowable force on the electrode within a range of 5g-50g, selecting a maximum allowable temperature, of tissue to be ablated, within a range of 55° C.-65° C., and selecting an irrigation rate for providing irrigation fluid to the electrode within a range of 8-45 ml/min. The method further includes performing an ablation of tissue using the selected values by initially using the first power, switching to the second power after a predefined time between 3 s and 6 s, and terminating the ablation after a total time for the ablation between 10 s and 20 s.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/179,167, filed Jun. 10, 2016, now U.S. Pat. No. 10,441,354 whichclaims the benefit of U.S. Provisional patent application No.62/286,534, filed Jan. 25, 2016, the entire contents both of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to surgery, and specifically tosurgery using radiofrequency ablation.

BACKGROUND OF THE INVENTION

Radiofrequency (RF) ablation is a treatment modality that kills unwantedtissue by heat. Starting with cardiac arrhythmia treatment in the 1980s,RF ablation has found clinical application in a number of diseases, andis now the treatment of choice for certain types of cardiac arrhythmia,and certain cancers. During RF ablation, an electrode is inserted intoproximity with the target region under medical imaging guidance. Tissuesurrounding the electrode in the target region is destroyed by heatingvia RF electric current.

RF ablation is typically performed at continuous power levels of theorder of 20-50 watts, with a contact force of approximately 10 g, andunder irrigation. The time of ablation, depending on the size of thelesion to be achieved, is typically approximately 1 minute. In general,higher power levels reduce the time needed for forming a specificlesion. However, in prior art systems large values of continuous powercannot be used because of the danger of steam pops being formed.

U.S. Patent Publication 2010/0057072, to Roman et al., whose disclosureis incorporated herein by reference, describes an ablation catheter forperforming tissue ablation. The disclosure states that RF energy may besafely delivered potentially at wattages up to 100 W.

U.S. Pat. No. 7,207,989, to Pike Jr. et al., whose disclosure isincorporated herein by reference, describes a method for ablating tissuein or around the heart to create an enhanced lesion. The distal end of aneedle electrode is introduced into the tissue. Anelectrically-conductive fluid is infused through the needle electrodeand into the tissue. The tissue is ablated after and/or duringintroduction of the fluid into the tissue.

U.S. Patent Publication 2015/0272655, to Condie et al., whose disclosureis incorporated herein by reference, describes a system for preventingunintended tissue damage from the delivery of unintended bipolarradiofrequency energy. The disclosure states that if 100 watts of RFenergy is being delivered but only 10 watts is required to produce adesired electrode temperature, an electrode may be activated for 10% ofa given period of time and deactivated for 90% of that duration of time.

U.S. Pat. No. 8,641,705, to Leo et al., whose disclosure is incorporatedherein by reference, describes an apparatus for controlling lesion sizein catheter-based ablation treatment. The apparatus measures the forceexerted by a contact ablation probe on a target tissue and integratesthe force over an energization time of the ablation probe. Theforce-time integral can be calculated and utilized to provide anestimated lesion size (depth, volume and/or area) in real time.

U.S. Pat. No. 8,882,761, to Desai, whose disclosure is incorporatedherein by reference, describes a catheter for ablation. The disclosurerefers to commonly practiced ablation procedure, and states that in sucha procedure 35 to 50 watts of power is delivered at 40 to 50 degreeCelsius through a temperature controlled Radiofrequency Generator, andthat the saline irrigation fluid rate during the ablation is 30 ml/min.

U.S. Patent Publication 2011/0009857, to Subramaniam et al., whosedisclosure is incorporated herein by reference, describes anopen-irrigated catheter with turbulent flow. Pressurized fluid isdelivered from a fluid lumen of a catheter body into an ablationelectrode. Fluid flow in the fluid lumen is generally laminar. Thegenerally laminar fluid flow is transformed from the fluid lumen into aturbulent fluid flow within the ablation electrode.

In an article by Topp et al., entitled “Saline-linked surfaceradiofrequency ablation: Factors affecting steam popping and depth ofinjury in the pig liver,” Ann. Surg., vol. 239, no. 4, pp. 518-27(2004), the authors claim to have determined parameters that predictsteam popping, and depth of tissue destruction under nonpoppingconditions. The article is incorporated herein by reference.

Documents incorporated by reference in the present patent applicationare to be considered an integral part of the application except that, tothe extent that any terms are defined in these incorporated documents ina manner that conflicts with definitions made explicitly or implicitlyin the present specification, only the definitions in the presentspecification should be considered.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a method, including:

selecting a maximum radiofrequency (RF) power to be delivered by anelectrode within a range of 70 W-100 W;

selecting an allowable force on the electrode within a range of 5 g-50g;

selecting a maximum allowable temperature, of tissue to be ablated,within a range of 55° C.-65° C.;

selecting an irrigation rate for providing irrigation fluid to theelectrode within a range of 8-45 ml/min; and

performing an ablation of the tissue using the selected values.

In a disclosed embodiment the selected values are: maximum RF power 90W, allowable force between 10 g and 20 g, maximum allowable temperature60° C., and irrigation rate 15 ml/min, and the power is delivered for 3s so as to provide a lesion having a depth between 1 mm and 3 mm.

In a further disclosed embodiment the selected values are: maximum RFpower 90 W, allowable force between 10 g and 20 g, maximum allowabletemperature 60° C., and irrigation rate 15 ml/min, and the power isdelivered for 3 s, and is then reduced to 50 W so as to provide a lesionhaving a depth between 4 mm and 5 mm.

In a yet further disclosed embodiment the method includes measuring animpedance to an RF power delivered by the electrode during the ablation,and, when a change in the impedance exceeds a preset value, halting theablation of the tissue. Typically, the change is at least 7Ω.

In an alternative embodiment the method includes measuring at respectivetimes a temperature of the tissue, and, when the temperature exceeds theselected maximum allowable temperature, reducing a level of an RF powerdelivered by the electrode. Typically, the temperature is measured at afrequency of at least 30 Hz.

There is further provided, according to an embodiment of the presentinvention, a method, including:

selecting a first maximum radiofrequency (RF) power to be delivered byan electrode within a range of 70 W-100 W;

selecting a second maximum RF power to be delivered by the electrodewithin a range of 20 W-60 W;

selecting an allowable force on the electrode within a range of 5 g-50g;

selecting a maximum allowable temperature, of tissue to be ablated,within a range of 55° C.-65° C.;

selecting an irrigation rate for providing irrigation fluid to theelectrode within a range of 8-45 ml/min; and

performing an ablation of tissue using the selected values by initiallyusing the first power, switching to the second power after a predefinedtime between 3 s and 6 s, and terminating the ablation after a totaltime for the ablation between 10 s and 20 s.

In a disclosed embodiment the method includes measuring an impedance toan RF power delivered by the electrode during the ablation, and, when achange in the impedance exceeds a preset value, halting the ablation ofthe tissue. Typically, the change is at least 7Ω.

In a further disclosed embodiment the method includes measuring atrespective times a temperature of the tissue, and, when the temperatureexceeds the selected maximum allowable temperature, reducing a level ofan RF power delivered by the electrode. Typically the temperature ismeasured at a frequency of at least 30 Hz.

There is further provided, according to an embodiment of the presentinvention, a method, including:

performing an ablation procedure on biological tissue usingradiofrequency (RF) power;

measuring an impedance to the RF power during the procedure; and

when a change in the impedance exceeds a preset value, halting supply ofthe RF power to the tissue. Typically, the change is at least 70.

There is further provided, according to an embodiment of the presentinvention, a method, including:

performing an ablation procedure on biological tissue usingradiofrequency (RF) power;

measuring at respective times a temperature of the tissue; and

when the temperature exceeds a preset maximum allowable temperature,reducing a level of the RF power supplied to the tissue. Typically, thetemperature of the tissue is measured at a frequency of at least 30 Hz.

There is further provided, according to an embodiment of the presentinvention, apparatus, including:

an electrode;

a power control module configured to select a maximum radiofrequency(RF) power to be delivered by the electrode within a range of 70 W-100W; and

a processor coupled to the power control module and configured to:

select an allowable force on the electrode within a range of 5 g-50 g;

select a maximum allowable temperature, of tissue to be ablated, withina range of 55° C.-65° C.;

select an irrigation rate for providing irrigation fluid to theelectrode within a range of 8-45 ml/min; and

perform an ablation of the tissue using the selected values.

There is further provided, according to an embodiment of the presentinvention, apparatus, including:

an electrode;

a power control module configured to select a first maximumradiofrequency (RF) power to be delivered by the electrode within arange of 70 W-100 W and to select a second maximum RF power to bedelivered by the electrode within a range of 20 W-60 W; and

a processor coupled to the power control module and configured to:

select an allowable force on the electrode within a range of 5 g-50 g;

select a maximum allowable temperature, of tissue to be ablated, withina range of 55° C.-65° C.;

select an irrigation rate for providing irrigation fluid to theelectrode within a range of 8-45 ml/min; and

perform an ablation of tissue using the selected values by initiallyusing the first power, switching to the second power after a predefinedtime between 3 s and 6 s, and terminating the ablation after a totaltime for the ablation between 10 s and 20 s.

There is further provided, according to an embodiment of the presentinvention, apparatus, including:

a power control module configured to perform an ablation procedure onbiological tissue using radiofrequency (RF) power; and

a processor configured to:

measure an impedance to the RF power during the procedure; and

when a change in the impedance exceeds a preset value, halt supply ofthe RF power to the tissue.

There is further provided, according to an embodiment of the presentinvention, apparatus, including:

a power control module configured to perform an ablation procedure onbiological tissue using radiofrequency (RF) power; and

a processor configured to:

measure at respective times a temperature of the tissue; and

when the temperature exceeds a preset maximum allowable temperature,reduce a level of the RF power supplied to the tissue.

The present disclosure will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an ablation system, according toan embodiment of the present invention;

FIGS. 2A, 2B, 2C, and 2D schematically illustrate a distal end of aprobe used in the system, according to an embodiment of the presentinvention; and

FIG. 3 is a flowchart of steps performed during an ablation sessionusing the system.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Radiofrequency (RF) ablation in prior art systems is typically performedat continuous power levels of the order of 20-50 watts, with a contactforce of approximately 10 g, and under irrigation. The time of ablation,depending on the size of the lesion to be achieved, is typicallyapproximately 1 minute. In general, higher power levels reduce the timeneeded for forming a specific lesion. However, in prior art systemslarge values of continuous power, of approximately 100 watts, cannot beused because of the danger of steam pops being formed.

The inventors have found that there is a range of values of contactforce and irrigation rate that allows continuous power of approximately100 watts to be applied, and that within this range of values, a “sweetspot,” no steam pops are formed. Application of this higher continuouspower reduces the time required to form a given lesion.

For example, in a disclosed embodiment, a first RF power to be deliveredto an electrode performing an ablation is selected to be in a range of70 W-100 W, and a second RF power for the electrode is selected to be ina range of 20 W-60 W. An allowable contact force on the electrode isselected to be in a range of 5 g-50 g, a maximum allowable temperatureof tissue to be ablated is selected to be in a range of 55° C.-65° C.,and an irrigation rate for providing irrigation fluid to the electrodeis selected within a range of 8-45 ml/min.

A lesion may be formed in the tissue using the selected values byinitially using the first power, switching to the second power after apredefined time between 3 s and 6 s, and terminating the ablation aftera total time for the ablation between 10 s and 20 s.

In embodiments of the present invention, during an ablation procedurethe temperature of the tissue being ablated is carefully monitored andrecorded at an extremely high rate. If the monitored temperature exceedsa preset maximum temperature limit, the RF power supplied to the tissueis reduced.

The impedance to the RF energy supplied to the tissue being ablated isalso monitored. If the impedance increases by more than a preset value,the RF energy supply is halted.

The monitoring of the temperature and of the impedance allowsembodiments of the present invention to perform tissue ablations atpowers up to 100 W without adverse effects on the tissue during theablation session. The high powers enable the ablation session to beshortened to times typically of no more than 10 s.

DETAILED DESCRIPTION

Reference is now made to FIG. 1, which is a schematic illustration of aninvasive medical procedure using an ablation apparatus 12, according toan embodiment of the present invention. The procedure is performed by aphysician 14, and, by way of example, the procedure in the descriptionherein below is assumed to comprise ablation of a portion of amyocardium 16 of the heart of a human patient 18. However, it will beunderstood that embodiments of the present invention are not justapplicable to this specific procedure, and may include substantially anyablation procedure on biological tissue.

In order to perform the ablation, physician 14 inserts a probe 20 into alumen of the patient, so that a distal end 22 of the probe enters theheart of the patient. Distal end 22 comprises one or more electrodes 24mounted on the outside of the distal end, the electrodes contactingrespective locations of the myocardium. Probe 20 has a proximal end 28.Distal end 22 of the probe is described in more detail below withreference to FIGS. 2A, 2B, 2C and 2D.

Apparatus 12 is controlled by a system processor 46, which is located inan operating console 48 of the apparatus. Console 48 comprises controls49 which are used by physician 14 to communicate with the processor.During the procedure, processor 46 typically tracks a location and anorientation of distal end 22 of the probe, using any method known in theart. For example, processor 46 may use a magnetic tracking method,wherein magnetic transmitters external to patient 18 generate signals incoils positioned in the distal end. The Carto® system produced byBiosense Webster, of Diamond Bar, Calif., uses such a tracking method.

The software for processor 46 may be downloaded to the processor inelectronic form, over a network, for example. Alternatively oradditionally, the software may be provided on non-transitory tangiblemedia, such as optical, magnetic, or electronic storage media. The trackof distal end 22 is typically displayed on a three-dimensionalrepresentation 60 of the heart of patient 18 on a screen 62. Theprogress of the ablation performed with apparatus 12 is typically alsodisplayed on screen 62, as a graphic 64 and/or alphanumeric data 66.

In order to operate apparatus 12, processor 46 communicates with amemory 50, which has a number of modules used by the processor tooperate the apparatus. Thus, memory 50 comprises a temperature module52, a power control module 54, a force module 56, and an irrigationmodule 58, the functions of which are described below. The modules maycomprise hardware as well as software elements.

FIGS. 2A, 2B, 2C, and 2D schematically illustrate distal end 22 of probe20, according to an embodiment of the present invention. FIG. 2A is asectional view along the length of the probe, FIG. 2B is across-sectional view along a cut IIB-IIB that is marked in FIG. 2A, FIG.2C is a perspective view of a section of the distal end and FIG. 2D is aschematic cross-sectional view of a force sensor 90 incorporated into aproximal portion 92 of the distal end. An insertion tube 70 extendsalong the length of the probe and is connected at the termination of itsdistal end to a conductive cap electrode 24A, which is used forablation. Conductive cap electrode 24A is herein also termed theablation electrode. Cap electrode 24A has an approximately planeconducting surface 84 at its distal end and a substantially circularedge 86 at its proximal end. Proximal to ablation electrode 24A thereare typically other electrodes such as an electrode 24B. Typically,insertion tube 70 comprises a flexible, biocompatible polymer, whileelectrodes 24A, 24B comprise a biocompatible metal, such as gold orplatinum, for example. Ablation electrode 24A is typically perforated byan array of irrigation apertures 72. In one embodiment there are 36apertures 72, distributed evenly over electrode 24A.

An electrical conductor 74 conveys radio-frequency (RF) electricalenergy from ablation module 54 (FIG. 1), through insertion tube 70, toelectrode 24A, and thus energizes the electrode to ablate myocardialtissue with which the electrode is in contact. As described below,module 54 controls the level of RF power dissipated via electrode 24A.During the ablation procedure, irrigation fluid flowing out throughapertures 72 irrigates the tissue under treatment, and the rate of flowof fluid is controlled by irrigation module 58. The irrigation fluid isdelivered to electrode 24A by a tube (not shown in the diagram) withininsertion tube 70.

Temperature sensors 78 are mounted within conductive cap electrode 24Aat locations that are arrayed around the distal tip of the probe, bothaxially and circumferentially. In a disclosed embodiment consideredherein, cap 24A contains six sensors, with one group of three sensors ina distal location, close to the tip, and another group of three sensorsin a slightly more proximal location. This distribution is shown only byway of example, however, and greater or smaller numbers of sensors maybe mounted in any suitable locations within the cap. Sensors 78 maycomprise thermocouples, thermistors, or any other suitable type ofminiature temperature sensor. Sensors 78 are connected by leads (notshown in the diagram) running through the length of insertion tube 70 toprovide temperature signals to temperature module 52.

In a disclosed embodiment cap 24A comprises a side wall 73 that isrelatively thick, on the order of 0.5 mm thick, in order to provide thedesired thermal insulation between temperature sensors 78 and theirrigation fluid inside a central cavity 75 of the tip. The irrigationfluid exits cavity 75 through apertures 72. Sensors 78 are mounted onrods 77, which are fitted into longitudinal bores 79 in side wall 73.Rods 77 may comprise a suitable plastic material, such as polyimide, andmay be held in place at their distal ends by a suitable glue 81, such asepoxy. U.S. Patent Publication 2014/0171821, to Govari et al., whosedisclosure h is incorporated herein by reference, describes a catheterhaving temperature sensors mounted in a similar configuration to thatdescribed above. The arrangement described above provides an array ofsix sensors 78, but other arrangements, and other numbers of sensors,will be apparent to those having ordinary skill in the art, and all sucharrangements and numbers are included within the scope of the presentinvention.

In the description herein, distal end 22 is assumed to define a set ofxyz orthogonal axes, where an axis 94 of the distal end corresponds tothe z axis of the set. For simplicity and by way of example, the y axisis assumed to be in the plane of the paper, the xy plane is hereinassumed to correspond to the plane defined by circle 86, and the originof the xyz axes is assumed to be the center of the circle.

FIG. 2D is a schematic, sectional view of force sensor 90, according toan embodiment of the present invention. Sensor 90 comprises a spring 94,herein assumed to comprise a plurality of helices 96, connecting cap 24Ato proximal end 92. A position sensor 98 is fixed to the distal side ofspring 94, and is herein assumed to comprise one or more coils coupledby conductors 100 to force module 56.

An RF transmitter 102, typically a coil, is fixed to the proximal sideof spring 94, and the RF energy for the transmitter is provided fromforce module 56 via conductors 104. The RF energy from the transmittertraverses sensor 98, generating a corresponding signal in conductors 100of the sensor.

In operation, as force is exerted on cap 24A sensor 98 moves relative totransmitter 102, and the movement causes a change in the signals of thesensor. Force module 56 uses the change in signal of the sensor toprovide a metric of the force on cap 24A. The metric typically providesthe force in magnitude and direction.

A more detailed description of a sensor similar to sensor 90 is providedin U.S. Patent Publication 2011/0130648, which is incorporated herein byreference.

Returning to FIG. 1, temperature module 52 receives signals from the sixsensors 78 within cap 24A, and uses the signals to determine a maximumvalue of the six measured temperatures. The temperature module isconfigured to calculate the maximum temperature at a fixed rate, hereinassumed to be every 33 ms, but other embodiments may calculate themaximum temperature at higher or lower rates. In some embodiments themaximum temperature is determined at a frequency of at least 30 Hz. Thecalculated maximum temperature is herein also termed the measuredtemperature, and the measured temperature is registers the temperatureof the tissue being ablated. The temperature module passes the measuredtemperature value to power control module 54.

Power control module 54 provides RF power to cap 24A in a range of 1 Wto 100 W. In embodiments of the present invention the module can beconfigured to provide a maximum RF power to cap 24A that can be setwithin a range of 70 W-100 W. In some embodiments, the module can beconfigured to provide a further RF power to cap 24A in a different rangefrom the maximum. In one embodiment the further power range is 20 W-60W, and the further power is typically provided after the maximum power.The maximum RF power and the further RF power are also termed herein thefirst power and the second power.

The power control module also measures an impedance of cap 24A. Theimpedance is measured at a predefined rate, herein assumed to be every500 ms, but other embodiments may measure the impedance at a lower orhigher rate.

The maximum power, and the time period for which the power is delivered,is selected by physician 14. The physician may also select values of thepower less than 70 W, and corresponding time periods for delivery ofthis reduced power. The actual power delivered is determined by themeasured temperature received from temperature module 52, as describedbelow.

Typically, during an ablation session, the impedance of cap 24Adecreases. Embodiments of the present invention also check if theimpedance increases from a previous impedance measurement by more than apre-set value, herein assumed to be 7Ω, although other embodiments mayuse larger or smaller values of impedance increase for the pre-setvalue. An increase of impedance typically occurs if there is an unwantedchange in the tissue being ablated, such as charring or steam popping.If the impedance increases by more than the pre-set value, the powercontrol module is configured to stop the RF delivery to cap 24A.

Notwithstanding the powers selected by the physician, the power controlmodule is configured to reduce the power delivered, typically by betweenapproximately 5% and approximately 95%, if the measured temperaturereceived from the temperature module reaches or exceeds a maximumallowable temperature that is set by physician 14.

In one embodiment, power that has been originally set to 90 W is reducedto 50 W after 4 s, regardless of the readings from sensors 78. In anembodiment of the present invention, the maximum allowable temperaturemay be set within a range 60° C.-65° C. Typically, exceeding the maximumallowable temperature causes undesirable effects such as charring,coagulation on cap 24A, and/or steam pops in the tissue being ablated.

As explained above, force module 56 is able to measure the force on cap24A. In an embodiment, the allowable force for an ablation is in therange of 5 g-35 g.

Irrigation module 58 governs the rate at which irrigation fluid isdelivered to the catheter tip. In some embodiments of the presentinvention it may be set within the range of 8-45 ml/min.

FIG. 3 is a flowchart of steps performed in operation of apparatus 12during an ablation session, according to an embodiment of the presentinvention. In an embodiment of the present invention, an ablationsession comprises two time periods: a first time period during which afirst target power applies, followed by a second time period duringwhich a second target power applies. In some ablation sessions only thefirst time period is used, and in this case there is only one targetpower set. The target powers within each time period are maximum RFpowers which may be delivered by power control module 54.

In a range setting step 200, ranges for each of the variable parametersreferred to above are set. In one embodiment the ranges are set as shownin Table I. Typically, for the target powers, an operator of the systemonly sets the first target power, while the second power isautomatically set by the system.

TABLE I Parameter Range First Target Power 70 W-100 W Second TargetPower 20 W-60 W  Allowable Force 5 g-50 g Maximum allowable temperature55° C.-65° C.  Irrigation rate     8-45 ml/min First Time Period (duringwhich First 1 s to 6 s Target Power is operative) Second Time Period(during which Up to 14 s Second Target Power is operative) Overall TimePeriod for Power 1 s-20 s Delivery (Sum of First and Second TimePeriods)

Range setting step 200 is implemented before physician 14 performs anablation.

At the beginning of an ablation session, in a probe introduction step202, physician 14 inserts probe 20 into a desired location in myocardium16, using the tracking system incorporated into apparatus 12.

In a select value step 204, prior to performing the ablation procedure,physician 14 selects values of the parameters listed in Table I that areto be used in the procedure, and uses controls 49 to provide the valuesto the system. Alternatively, the physician selects a predetermined setof the values of the parameters listed in Table I, typically by choosinga “recipe,” comprising the values, from a group of such recipes. Theselected values typically depend on the depth of lesion it is desired toform by the procedure. For lesions of 1-3 mm depth the inventors havefound that the values of the parameters given by Table II give goodresults. For lesions of 4-5 mm depth the inventors have found that thevalues of the parameters given by Table III give good results.

TABLE II Lesions of 1-3 mm Depth Parameter Value First Target Power 90 WSecond Target Power Not set Allowable Force 10 g-20 g Maximum allowabletemperature 60° C. Irrigation rate 15 ml/min First Time Period 3 sSecond Time Period Not set

TABLE III Lesions of 4-5 mm Depth Parameter Value First Target Power 90W Second Target Power 50 W Allowable Force 10 g-20 g Maximum allowabletemperature 60° C. Irrigation rate 15 ml/min First Time Period 3 sSecond Time Period 7 s

Those having ordinary skill in the art will be able to determine, forother lesion depths, required values of the parameters within the rangesgiven by Table I, without undue experimentation.

In a begin RF delivery step 206, the first time period of the ablationsession begins, by physician 14 initiating operation of apparatus 12.The ablation session uses the parameter values selected in step 204, inorder to perform the ablation. Typically, during the ablation session,screen 62 displays values of the parameters listed in Table I to thephysician. Screen 62 may also be configured to display to the physician,by methods which are known in the art, the progress of the RF delivery.The display of the progress may be graphical, such as a simulation ofthe dimensions of the lesion as it is produced by the ablation, and/oralphanumeric.

The remaining steps of the flowchart apply for both the first timeperiod, and, if operative, for the second time period.

During the RF delivery procedure the system uses the power controlmodule to perform a number of checks on the progress of the procedure,as shown in the flowchart by decision steps 208, 210, and 214.

In step 208, the power control module checks if the impedance to thedelivered RF power of cap 24A has increased by more than the pre-setimpedance value. If it has, the system halts the procedure in atermination step 216. If step 208 returns a negative value, control ofthe flowchart continues to decision step 210.

In step 210, the power control module checks if the measured temperatureexceeds or reaches the pre-set maximum allowable temperature selected instep 204. If decision step 210 returns a positive value, the powercontrol module, in a power reduction step 218, reduces the power to cap24A.

The power reduction in step 218 is a function of a number of parameters:

A difference in temperature between the maximum allowable temperature T(set in step 204) and the measured temperature T_(t) at a time t,

A change of measured temperatures between sequential temperaturemeasurements, i.e., T_(t-1)−T_(t),

A target power P, where if the flowchart is functioning in the firsttime period, P is the first target power, and if the flowchart isfunctioning in the second time period, P is the second target power, and

A power P_(t) measured at time t.

In one embodiment the following equations applies for the powerreduction:

$\begin{matrix}{{\Delta\;{P(T)}} = {\frac{a\left( {T_{t - 1} - T_{t}} \right)}{T} + \frac{b\left( {T - T_{t}} \right)}{T}}} & (1)\end{matrix}$where ΔP(T) is a fractional change in power as a function oftemperature, and a and b are numerical constants. In a disclosedembodiment a=10 and b=1.

$\begin{matrix}{{\Delta\;{P(p)}} = \frac{\left( {P - P_{t}} \right)}{P}} & (2)\end{matrix}$where ΔP(p) is a fractional change in power as a function of power.ΔP=min(ΔP(T),ΔP(p))  (3)where min(ΔP(T), ΔP(p)) is the minimum of ΔP(T) and ΔP(p), and ΔP is thefractional change in power applied in step 218.

Typically, power reduction step 218 is performed reiteratively withdecision step 210, until the measured temperature is below the presetmaximum temperature.

If step 210 returns a negative value, control continues to decision step214.

In decision step 214, the system checks if the overall time period forthe ablation session, set in step 204, has been reached. If it has, thenthe flowchart ends. In decision step 214, the system also checks if theend of the first time period has been reached, and if it has, the systementers the second time period.

If the overall time period has not been reached, control passes to acontinuing ablation step 222, where the system continues the ablation,and returns to decision steps 208, 210, and 214. Decision steps 208,210, and 214 have been presented sequentially in the flowchart forsimplicity and clarity. Typically, however, the system uses the powercontrol module to perform the steps in parallel.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. A method, comprising: selecting a firstmaximum radiofrequency (RF) power of 90 W to be delivered to biologicaltissue by an electrode; selecting a second maximum RF power of 50 W tobe delivered to the biological tissue by the electrode; defining apreset impedance value I; performing an ablation procedure on thebiological tissue using radiofrequency (RF) power at the first maximumRF power; measuring an impedance I_(T-1) to the first maximum RF powerduring the ablation procedure at time (T-1); measuring an impedanceI_(T) to the first maximum RF power during the ablation procedure attime (T); and when a difference between I_(T) and I_(T-1) exceeds thepreset impedance value I, halting supply of the first maximum RF powerto the biological tissue.
 2. The method of claim 1, wherein the presetimpedance value I is 7 ohms.
 3. The method of claim 2, furthercomprising measuring temperature of the biological tissue when thedifference between I_(T) and I_(T-1) is not greater than the presetimpedance value I.
 4. The method of claim 3, wherein the measuringtemperature of the biological tissue occurs at a frequency of at least30 Hz.
 5. The method of claim 1, further comprising reducing supply ofthe RF power to the biological tissue from the first maximum RF power tothe second maximum RF power when a measured temperature of thebiological tissue reaches or exceeds a preset maximum temperature value.6. The method of claim 1, further comprising selecting an irrigationrate for providing irrigation fluid to the electrode within a range of8-45 ml/min.
 7. The method of claim 1, further comprising delivering thefirst maximum RF power for 4 s, and delivering the second maximum RFpower for between 6 s and 16 s.
 8. The method of claim 1, furthercomprising measuring the impedances I_(T) and I_(T-1) at a rate of aboutevery 500 ms.
 9. Apparatus, comprising: a power control moduleconfigured to perform an ablation procedure on biological tissue usingradiofrequency (RF) power; and a processor configured to: select a firstmaximum radiofrequency (RF) power of 90 W to be delivered to thebiological tissue by an electrode; select a second maximum RF power of50 W to be delivered to the biological tissue by the electrode; define apreset impedance value I; perform an ablation procedure on thebiological tissue using radiofrequency (RF) power at the first maximumRF power; measure an impedance I_(T-1) to the first maximum RF powerduring the procedure at time (T-1); measure an impedance I_(T) to thefirst maximum RF power during the procedure at time (T); and whendifference between I_(T) and I_(T-1) exceeds a preset impedance value,halt supply of the first maximum RF power to the biological tissue. 10.The apparatus of claim 9, wherein the preset impedance value is 7 ohms.11. The apparatus of claim 10, wherein the processor is configured tomeasure temperature of the tissue when the difference between I_(T) andI_(T-1) is not greater than the preset impedance value.
 12. Theapparatus of claim 11, wherein the processor is configured to measuretemperature of the biological tissue at a frequency of at least 30 Hz.13. The apparatus of claim 9, wherein the processor is configured toreduce supply of the RF power to the biological tissue from the firstmaximum RF power to the second maximum RF power when a measuredtemperature of the biological tissue reaches or exceeds a preset maximumtemperature value.
 14. The apparatus of claim 9, wherein the processoris further configured to select an irrigation rate for providingirrigation fluid to the electrode within a range of 8-45 ml/min.
 15. Theapparatus of claim 9, wherein the processor is further configured todeliver the first maximum RF power for 4 s, and deliver the secondmaximum RF power for between 6 s and 16 s.
 16. The apparatus of claim 9,wherein the processor is configured to measure the impedances I_(T) andI_(T-1) at a rate of about every 500 ms.